This study will examine how objective and perceived measures of neighborhood safety influence young children's physical activity, sedentary behavior, and body mass index (BMI) using longitudinal models estimated on a nationally representative sample of kindergartners. Our study has three specific aims: 1. Examine the correlation between perceived and objective measures of neighborhood safety. 2. Estimate panel data models to examine whether changes in perceived and objective measures of neighborhood safety influence children's physical activity, sedentary behavior, and BMI. 3. Examine whether the influence of perceived and objective measures of neighborhood safety physical activity, sedentary behavior, and BMI vary by children's socio-economic characteristics. We will use data from the Early Childhood Longitudinal Study - Kindergarten Class (ECLS-K), a nationally representative sample of kindergartners with follow-up data in the first, third, fifth and eighth grades. Physical activity, inactivity, BMI and perceived measures of neighborhood safety (parent, school administrator, and interviewer reports) are available in multiple waves in the ECLS-K. Objective measures of neighborhood safety will be constructed by linking each year of the ECLS-K to county level crime data from the Uniform Crime Reports and census tract level crime data from a convenience sample of individual police departments. Our analytical approach relies on child fixed effects models that indentify the effects of neighborhood safety by leveraging changes in the dependent variables and neighborhood safety over time for the same child. Results will be disaggregated by key socioeconomic characteristics in order to understand their interaction with neighborhood safety. In addition, comparison of results using perceived and objective measures of neighborhood safety from multiple perspectives and various levels of geographic precision will be useful in disentangling the importance of this integral aspect of the social environment. Public Health Relevance: This study can inform policy interventions that address neighborhood safety and the physical activity behaviors of young children and how such interventions may be influenced by socio-economic characteristics. The study also provides greater insight on parental decision-making by exploiting a rich dataset of perceived and objective safety measures. Similar to school-based physical activity interventions, neighborhood safety programs have the ability to affect large numbers of children making them a potentially important part of a broader public health strategy.